1. Chapter 9 Day 2 Tests About a Population Proportion

2. Yesterday we talked about the logic behind significance tests. ◦ Suppose the mean score on the SSHA is 115 and the standard deviation is 30. A teacher suspects that older students have better attitudes about school, so she gives the SSHA to 25 students who are at least 30 years old.  What is the sampling distribution of x-bar if H o is true?  Suppose the sample data give an x-bar of 118.6. Sketch the curve and shade the p-value.  Suppose the sample data give an x-bar of 125.7. Shade this p-value.  Which of the results would give more evidence against H o ?

3. Today… Today, we’ll conduct hypothesis tests in detail. In addition, we’ll talk about α (alpha), and discuss fixed levels of significance.

4. This is Greek to me! The Greek letter alpha, α, is called the significance level. If the p-value is at or below the significance level, we consider the data to be statistically significant. This is often 0.05, but other times you will see other levels given to you.

5. Steps for a Hypothesis Test Step 1: Write the null and alternative hypotheses. ALWAYS use parameter symbols in the hypotheses. Step 2: Choose the appropriate inference procedure. Check the conditions of the test. (They will match the conditions for confidence interval). Step 3: Identify the test by name OR by formula. Calculate the test statistic and find the p-value. Be sure you graph and shade here. Step 4: Link the p-value to the conclusion and write the conclusion in the context of the problem. Concluding sentences: Because the p-value of ____ is [ ] an α of ___, we [reject / fail to reject] the null hypothesis that [context]. We [find / fail to find] evidence to suggest that the alternative hypothesis [context] is true.

6. Carrying Out a Significance Test Tests About a Population Proportion Recall our basketball player who claimed to be an 80% free-throw shooter. In an SRS of 50 free-throws, he made 32. His sample proportion of made shots, 32/50 = 0.64, is much lower than what he claimed. Does it provide convincing evidence against his claim? To find out, we must perform a significance test of H 0 : p = 0.80 H a : p < 0.80 where p = the actual proportion of free throws the shooter makes in the long run. Check Conditions: In Chapter 8, we introduced three conditions that should be met before we construct a confidence interval for an unknown population proportion: Random, Normal, and Independent. These same three conditions must be verified before carrying out a significance test. Random We can view this set of 50 shots as a simple random sample from the population of all possible shots that the player takes. Normal Assuming H 0 is true, p = 0.80. then np = (50)(0.80) = 40 and n (1 - p) = (50)(0.20) = 10 are both at least 10, so the normal condition is met. Independent In our simulation, the outcome of each shot does is determined by a random number generator, so individual observations are independent.

7. Carrying Out a Significance Test If the null hypothesis H 0 : p = 0.80 is true, then the player’s sample proportion of made free throws in an SRS of 50 shots would vary according to an approximately Normalsampling distribution with mean Test About a Population Proportion Calculations: Test statistic and P-value A significance test uses sample data to measure the strength of evidence against H 0. Here are some principles that apply to most tests: The test compares a statistic calculated from sample data with the value of the parameter stated by the null hypothesis. Values of the statistic far from the null parameter value in the direction specified by the alternative hypothesis give evidence against H 0. Definition: A test statistic measures how far a sample statistic diverges from what we would expect if the null hypothesis H 0 were true, in standardized units. That is

8. The One-Sample z Test for a Proportion The z statistic has approximately the standard Normal distribution when H 0 is true. P -values therefore come from the standard Normal distribution. Here is a summary of the details for a one-sample z test for a proportion. Tests About a Population Proportion Choose an SRS of size n from a large population that contains an unknown proportion p of successes. To test the hypothesis H 0 : p = p 0, compute the z statistic Find the P-value by calculating the probability of getting a z statistic this large or larger in the direction specified by the alternative hypothesis H a : Choose an SRS of size n from a large population that contains an unknown proportion p of successes. To test the hypothesis H 0 : p = p 0, compute the z statistic Find the P-value by calculating the probability of getting a z statistic this large or larger in the direction specified by the alternative hypothesis H a : One-Sample z Test for a Proportion

9. Example A potato-chip producer has just received a truckload of potatoes from its main supplier. If the producer determines that more than 8% of the potatoes in the shipment have blemishes, the truck will be sent away to get another load from the supplier. A supervisor selects a random sample of 500 potatoes from the truck. An inspection reveals that 47 of the potatoes have blemishes. Carry out a significance test at the α= 0.10 significance level. What should the producer conclude?

10. 2-sided alternatives When the alternative hypothesis is two- sided, finding the p-value is a little different. You don’t know which way you are looking. Because you shade on BOTH sides of the normal curve, you need to DOUBLE the p-value

11. According to the Centers for Disease Control and Prevention (CDC) Web site, 50% of high school students have never smoked a cigarette. Taeyeon wonders whether this national result holds true in his large, urban high school. For his AP Statistics class project, Taeyeon surveys an SRS of 150 students from his school. He gets responses from all 150 students, and 90 say that they have never smoked a cigarette. What should Taeyeon conclude? Give appropriate evidence to support your answer.

12. Confidence Intervals and Two-Sided Tests Tests About a Population Proportion There is a link between confidence intervals and two-sided tests. The 95% confidence interval gives an approximate range of p 0 ’s that would not be rejected by a two-sided test at the α = 0.05 significance level. The link isn’t perfect because the standard error used for the confidence interval is based on the sample proportion, while the denominator of the test statistic is based on the value p 0 from the null hypothesis. A two-sided test at significance level α (say, α = 0.05) and a 100(1 – α )% confidence interval (a 95% confidence interval if α = 0.05) give similar information about the population parameter. If the sample proportion falls in the “fail to reject H 0 ” region, like the green value in the figure, the resulting 95% confidence interval would include p 0. In that case, both the significance test and the confidence interval would be unable to rule out p 0 as a plausible parameter value. However, if the sample proportion falls in the “reject H 0 ” region, the resulting 95% confidence interval would not include p 0. In that case, both the significance test and the confidence interval would provide evidence that p 0 is not the parameter value.

13. Homework: Chapter 9 # 34, 41, 42, 47, 50